Better error message when "if name == main" guard is needed #16900
Description
Description & Motivation
When you run with DDP-spawn, and don't add the if __name__ == "__main__" guard at the entry point, the user gets this error:
RuntimeError:
An attempt has been made to start a new process before the
current process has finished its bootstrapping phase.
This probably means that you are not using fork to start your
child processes and you have forgotten to use the proper idiom
in the main module:
if __name__ == '__main__':
freeze_support()
...
The "freeze_support()" line can be omitted if the program
is not going to be frozen to produce an executable.
The error "looks" strange and the instructions how to fix it are a bit unclear. The user may be confused what "freeze_support" means.
Pitch
In the MultiProcessingLauncher, wrap the call with a try catch and re-raise a better error message, with Lightning-specific instructions of possible solutions (perhaps suggesting ddp instead of spawn).
Note: The issue will still independently exist if you don't run with ddp-spawn but have DataLoader(num_workers>0).
Alternatives
Keep as is. The error message and solutions are all documented.
Recently, we switched the default to ddp when devices>1 in Trainer/Fabric, which should mitigate the issue.
Additional context
No response